A Patent Document 1 listed below discloses a multi-joint robot arm so-called as a bending robot that takes out a plate-shaped workpiece placed on a stacker by suctioning or the like and then supplies it to a bending process at a bending machine. In addition, a Patent Document 2 listed below discloses a technology in which workpiece detection carried out when taking out one workpiece from stacked workpieces by a multi-joint robot arm is made based on an image(s) taken by a camera disposed above. In addition, a Patent Document 3 listed below discloses a technology in which, in order to suction an uppermost one of stacked plate-shaped workpieces and then convey it, detection of an uppermost position of the stacked workpieces is carried out by a non-contact type distance sensor, using a laser or the like, that is attached to a hanging bracket disposed above the workpieces.